In a typical fuel dispensing transaction, a customer arranges for payment, either by paying at the fuel dispenser with a credit card or debit card, or by paying a cashier. Next, a fuel nozzle is inserted into the fill neck of the vehicle, or other selected container, and fuel is dispensed. Displays on the fuel dispenser indicate how much fuel has been dispensed as well as a dollar value of the purchase. Dependent upon the timing and manner of payment for the fuel, either the customer terminates the flow of fuel into the vehicle by manually releasing the fuel nozzle, or the fuel dispenser automatically terminates the flow of fuel either at a pre-selected dollar amount or when the tank of the vehicle is full. In either case, the closing of the fuel valve within the fuel nozzle is herein referred to as a “nozzle snap event.”
During such operations, a series of valves are opened and closed along the fuel flow path within the fuel dispenser. Referring now to FIG. 1, a schematic of a typical prior art fuel dispenser 100 is shown. As shown, fuel is pumped from an underground storage tank 102 through a fuel pipe 104 to a flexible fuel hose 105 which terminates with a fuel nozzle 106 including a fuel valve 108. To initiate fuel flow, the customer manually activates a trigger on fuel nozzle 106 which opens fuel valve 108 so that fuel is dispensed into the vehicle. Fuel flow through fuel valve 108 is detected by a flow switch 116 which, as shown, is a one-way check valve that prevents rearward flow through fuel dispenser 100. Once fuel flow is detected, flow switch 116 sends a signal on communication line 124 to a control system 120. Control system 120 is typically a microprocessor, a microcontroller, or other electronics with associated memory and software programs. Upon receiving the flow initiation signal from flow switch 116, control system 120 starts counting the pulses from a pulser 118. The pulses are generated by the rotation of a fuel meter 114 and are directly proportional to the fuel rate being measured.
As is known, fuel dispensers keep track of the amount of fuel dispensed so that it may be displayed to the customer along with a running total of how much the customer will have to pay to purchase the dispensed fuel. This is typically achieved with fuel meter 114 and a pulser 118. When fuel passes through fuel meter 114, it rotates and pulser 118 generates a pulse signal, with a known number of pulses being generated per gallon of fuel dispensed. The number of pulse signals generated and sent to control system 120 on communication line 126 are processed to arrive at an amount of fuel dispensed and an associated cost to the customer. These numbers are displayed to the customer to aid in making fuel dispensing decisions. As well, control system 120 uses the information provided by fuel meter 114 to regulate the operation of valve 112 during fueling operations.
As shown, fuel dispenser 100 includes a turbine style fuel meter 114, such as that disclosed in U.S. Pat. No. 7,028,561, which is hereby incorporated by reference in its entirety. Flow switch 116 is used in conjunction with turbine fuel meter 114 since the possibility exists that the rotors (not shown) of fuel meter 114 can bind during use, yet still allow fuel to pass through the meter. As such, pulser 118 does not create pulses, and the flow of fuel can go undetected. However, fuel switch 116 detects fuel flow and sends a signal to control system 120, allowing control system 120 to detect the flow error. Other designs of non-positive displacement type fuel meters can be prone to this same issue.
Fuel flow through fuel nozzle 106 is terminated by a nozzle snap event, that event being caused either manually by the customer or automatically by fuel dispenser 100. As fuel valve 108 snaps shut, fuel flow through flow switch 116 begins to decrease and flow switch 116 begins to shut. As flow switch 116 shuts, it generates a signal that indicates to control system 120 that fuel flow is being terminated. In response, control system 120 disregards any additional pulse signals that are generated by pulser 118.
Potential inaccuracies may exist when attempting to determine the total volume of fuel dispensed from the typical fuel dispenser discussed above when nozzle snaps occur. A typical fuel supply pressure for fuel dispenser 100 is 30 pounds per square inch (psi) upstream of valve 112. As fuel is dispensed at increasing flow rates, the pressure differential between the fuel supply pressure and the fuel pressure at flow valve 108 increases. As shown in FIG. 2, a pressure differential of approximately 3 psi exists at a steady state flow rate of 2 gallons per minute (gpm), whereas at a flow rate of 10 gpm, the pressure differential is approximately 15 psi. When flow is terminated by a nozzle snap event, system pressure is equalized until fuel pressure along the entire fuel flow path is approximately equal to the supply pressure, in this case 30 psi. This occurs as fuel is added to the fuel flow path downstream of fuel meter 114 through flow switch 116.
The additional volume of fuel added downstream of fuel meter 114 as pressure is equalized within the system is not added to the total volume of fuel dispensed, as measured by the fuel meter, since flow switch 116 sends a signal to control system 120 at the occurrence of the nozzle snap event indicating that further pulses from the fuel meter should be ignored. The additional, undetected volume of fuel is then dispensed to the tank of the vehicle when fuel flow is reinitiated. As seen in FIG. 2, the volume of fuel required for system pressure equalization increases along with the increase in the pressure differential between the fuel supply pressure and the fuel pressure at fuel valve 108. Because the noted pressure differential increases as the flow rate at which fuel is dispensed increases, inaccuracies in measuring the total volume of fuel dispensed typically increase as the flow rate at which the fuel is being dispensed increases with nozzle snaps.